The present invention relates generally to the field of dental restorations and more particularly to porcelain crown restorations which utilize a thin metal foil as a substrate and to a method of making same.
Dental porcelain is commonly used in fabricating dental restorations. Porcelain restorations can be fabricated to approximate the natural tooth shading of the intended user and thereby provide a user with an aestheticly appealing replacement. Additionally, porcelain may be contoured to form a prescribed occlusal surface, is durable in saliva, has reduced thermal conductivity and resists abrasion from food and brushing.
Porcelain jacket crowns fell into disfavor for sometime because the use thereof required the dentist to grossly cut down a natural tooth and because the older feldspar porcelains tended to fracture. Improved formulations and methods for preparing porcelain crowns have since greatly enhanced the dependability and popularity of porcelain restorations.
One common method for preparing a porcelain jacket crown includes preparing a platinum foil matrix conforming to the shape which the restored tooth is to take and applying a porcelain cover, or jacket, onto the matrix. Typically, a porcelain jacket crown of this type will include a build-up of several layers of porcelain containing different concentrations of aluminum oxide (alumina). After the crown is formed to its "in-mouth" configuration, the platinum matrix is then removed from the crown before the crown is secured into the patient's mouth.
Another method for preparing porcelain crowns includes chemically bonding or fusing the porcelain to a metal substrate. For example, U.S. Pat. No. 4,273,580 is directed to a jacket crown formed of a composite body having an inner structure composed of a thin metal foil of platinum conforming in shape to the tooth preparation to be restored, at least one thin intermediate coating of a predetermined composition of finely divided particles bonded to the thin metal foil and a relatively thick fired-on outer coating of dental porcelain. The intermediate coating of the '580 patent comprises from about 1 to 100% by weight of a noble metal chloride selected from a specified group in combination with from 0 to 99% by weight of a gold based noble metal. According to the '580 patent, it is essential that the intermediate coating is bonded to the thin metal foil at an elevated sintering temperature of at least 1600.degree. F.
U.S. Pat. No. 4,392,829 is directed to a baked metal-porcelain dental restoration having a non-cast thin metal foil substrate. The surface of the foil substrate is textured and is capable of bonding directly to dental porcelain when baked without any intermediate interface compositions. According to the '829 patent, the porcelain layer is bonded directly to the textured surface and the exterior of the porcelain may be configured as prescribed. The metal substrate, however, is not sealed and the crown therefore is not fully protected against migration of fluids and contaminants.
U.S. Pat. No. 4,459,112 and its related patents, e.g. RE33,099, No. 4,492,579 and RE33,271, disclose jacket crown restorations wherein a metal foil substrate serves as an integral component of the finished porcelain jacket crown. The disclosed foil substrate is preferably circular in geometry and is composed of a thin foil metal base of platinum or another high fusing temperature metal and a thin coated layer superimposed on the base metal. The coated layer is indicated to be a noble metal base composition preferably with gold as its major constituent. The metal foil is folded into a predetermined geometrical shape having a multiple number of fold lines for forming a dental coping. The coping is adapted to a die and heat treated to sinter the overlapping folds to one another. In addition to requiring a special foil, this method produces multiple pleats (joint lines) in the substrate structure which are undesirable.